Bones and Muscles – Lung Anatomy

The main task of the lung (Latin: Pulmo) is the oxygenation of the blood and the elimination of carbon dioxide. This gas exchange takes place in the pulmonary alveoli (air sacs). Diseases of the lung are common and seen in every age group. In this context, asthma as a disease of children and teenagers, and the chronic obstructive pulmonary disease (COPD) as a disease of adults should be mentioned. Also, infectious diseases of the lung like pneumonia are part of the everyday-life in practices and hospitals. Anatomic knowledge of the lung is key to be able to classify the different clinical pictures. In the following article, you will get an overview over the location, the structure, and the functions of the lung.

00:00
The lungs are kept within a articulated skeletalcage. And this is important, because it protects
the lungs from damage, but also, it's importantfor function. Because it's expansion of the
skeletal cage that allows the lungs to expandand take air in from the atmosphere. And this
requires a fixed point. And there's a fixedpoint posteriorly, which is the thoracic vertebrae,
and there are 12 thoracic vertebrae. Eachis separated by cartilaginous intervertebral
discs, and the vertebrae are the site of thearticulation of the ribs, which curve around
from the vertebrae to meet with a fixed pointanteriorly, which is the sternum. And this
is a flat bone, which you can all feel inthe center of the anterior chest. And this
comprises of three parts: the manubrium, whichis the top part which joins to the clavicles,
which are the bones that allow the shouldersand the sternum to be joined together; the
sternal body, which is in the middle; andthere's a little process that sticks out the
bottom called the xiphoid process, and that'simportant because the diaphragm sticks onto
that. The clinical relevance of this bonystructure is that if you have several spinal
defects—a curvature of the spine, or scoliosis—thatwill affect the mechanics of breathing, because
that will affect the mechanics of expansionof the skeletal structures during respiration.
And that can actually lead to respiratoryfailure if severe scoliosis is present.
So the ribs are the bones that join the vertebraeand the sternum. And there are 12 pairs of
ribs arising from the thoracic vertebrae,one from each. They're made of bone posteriorly,
but anteriorly, they merge into cartilage,and it's these cartilages that form together
to form the costochrondral margin, which youcan feel between the abdomen and the thoracic
cavity on both sides, and also merge to formwith the sternum anteriorly (that's ribs 1
to 7). The clinical relevance of this is thatthe ribs articulate with the vertebrae, but
those specific joints can be affected by adisease called ankylosing spondylitis. And
if that disease is unchecked, then those articulationsbecome rigid, and the ribs will not move during
respiration. And again, that can lead to ventilatorydefects, although it's a relatively rare cause.
The 11th and 12th ribs are actually freestandingand are not directly involved in forming the
thoracic cavity. An important thing aboutthe ribs is that they… with each rib, there's
an accompanying blood vessel—a vein andan artery—and also a nerve. The nerve supplies
the skin overlying the rib. The vein and arterysupplies the tissues around that rib. They
all run together in a groove on the inferiorsurface of the rib, just inside. But it's
important to know about that, because anyprocedure which involves putting needles or
drains through the gaps between the ribs—anintercostal drain, for example—could penetrate
the artery and cause bleeding, which veryoccasionally can be fatal. So it's important
to know where those vessels run, and it'sjust underneath each rib.
So this is an overview of the skeletal cavityor the thoracic cavity. You can see the sternum
at the front of its body in the middle, themanubrium at the top, and the xiphoid process
at the bottom. And the vertebrae run behind,and then the ribs run in between the two.
To move the chest during respiration requiresmuscles. And there are three sets of muscles
involved. There are the muscles of the chestwall: These are the serratus anterior and
posterior, the trapezius of the neck, thepectoralis major and minor, rectus abdominis
(the abdominal muscles down here). They areall involved in respiration when you're having
forced respiration, such as when you're exercising,or if you have such severe lung disease that
you need to maximize your ventilatory capacity.
03:55
The second set of muscles that are important
are the intercostal muscles, and these aremuscles that run between each rib. And they're
divided into three categories, three types:there's external, which is on the outside;
the internal is in between; and then the subcostal,which is the most inferior (they are closest
to the parietal pleurae). And finally, thereis the diaphragm. And so the intercostals
are used for inspiration, and the diaphragmis used for inspiration as well. When you
have forced expiration, i.e. when you're exercising,you'll also recruit these muscles for that
process as well.
04:32
So to talk about the intercostals in a little
bit more detail: These cross between eachrib and are the main skeletal muscles of respiration.
So the external ones, which are the outerlayer—they run obliquely downwards anteriorly.
That means that when they contract, they liftthe ribs up and out and expand the chest.
And they are one of the main drivers of inspiration.
04:52
The internals run obliquely downwards posteriorly,
and they pull the ribcage back down againand therefore are used during forced expiration
but are not necessarily used… are not necessaryfor normal expiration. The subcostal muscles
run vertically, and they're in between…and they're the bottom layer, and they are
actually underneath the intercostal vein,artery, and nerve. The clinical relevance
of the intercostals is that if you have aproblem affecting the skeletal muscles, such
as motor neuron disease, then that can alsolead to a lung hypoventilation (underventilation
of the lung) and potentially respiratory failure.
05:29
And in fact, that is the mode of death for
many patients with end-stage muscle disease,such as motor neuron disease or muscular dystrophies.
This is a diagram, cross-section of the chestwall at the level of the diaphragm, and you
can see here the intercostals, which are atthe top part of this diagram with the external
intercostal on the outside, the middle layerbeing the internal, and the underneath layer
being the subcostal just next to the lung.
05:57
The diaphragm has that shape where it curves
up between the lung and the liver on thatside. On the left-hand side, it would be between
the lung and the spleen and the stomach. Andthen there's an extra layers of skin and subcutaneous
tissue and extrathoracic muscle outside ofthat.
The diaphragm itself is a smooth muscle. It'splaced under each lung, between the thoracic
cavity and the abdomen. It has a central tendinousarea, and it is essentially the main muscle
of respiration. When it contracts, it makesthe muscle smaller, and that pulls a domed
diaphragm flatter, and that will expand thelungs downwards. The diaphragm arises from
insertions on the xiphoid process anteriorly,the vertebrae back posteriorly, and the bottom
six ribs around each side. In general, theupper limit of the diaphragm is on the 5th
rib during… at rest and is slightly higheron the right-hand side than the left due to
the presence of the liver beneath it on theright-hand side.
Diaphragmatic movements and contractions arecontrolled by the phrenic nerve, which arises
from the cervical 3, 4, and 5 nerve roots.
07:06
That's important, because without that phrenic
nerve, the diaphragm will not move. If youhave a bilateral phrenic nerve palsy—for
example, if somebody has a cervical lesionabove C3, 4, and 5 at C2—then they will
die from respiratory failure unless there…some form of mechanical ventilation is used,
because neither diaphragm is moving. Thereare three openings in the diaphragm, and these
are important because they are sites of herniation.
07:29
So, for example, a hiatus hernia often comes
up through the opening through the esophagusand the vagus posteriorly behind the heart
and is visible on a chest x-ray. And the hiatushernia represents stomach tissue… the stomach
moving through the diaphragmatic opening intothe thoracic cavity. The other openings are
for the aorta and the inferior vena cava.
07:51
This is a diagram of the diaphragm. You can
see the dome of the right and the left hemidiaphragm.
07:56
The sort of slightly gray area in the middle
is the tendinous process. And you can seehow the diaphragm arises posteriorly from
the vertebrae of these insertions onto eachvertebral body from the costochondral margin
in the ribs around the side and the xiphoidprocess anteriorly. And this diagram clearly
shows that sort of… that the dome shapeof the diaphragm is, when it contracts, it
will shorten, and that dome will flatten.
08:19
And that's expands the lungs.

About the Lecture

The lecture Bones and Muscles – Lung Anatomy by Jeremy Brown, PhD is from the course Introduction to the Respiratory System.

Included Quiz Questions

Which of the following statements about the chest wall is FALSE?

There are two parts to the sternum.

There are twelve pairs of ribs.

The anterior cartilaginous ends of ribs 1 to 7 fuse with the sternum.

The intercostal artery and nerve run along the inferior aspect of each rib.

How many thoracic vertebrae are present in the body?

12.

9.

10.

14.

15.

The xiphoid process is the anatomical attachment point for which structure?

Diaphragm.

Facia.

Intercostal muscles.

Visceral plura.

What is scoliosis?

Excessive curvature of spine.

Poor medullary breathing signal.

Exaggerated medullary respiration signalling.

Straightening of spine.

Out curve in the spine.

Author of lecture Bones and Muscles – Lung Anatomy

Jeremy Brown, PhD

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